Basically what others had said. Also, banning gives you an idea about which heroes your opponent have or don't have.

Not to mention, you can play psychological warfare over ban phase and even bait opponents to pick certain heroes that you actually wanted them to pick (since your team may have a very strong direct counter to these heroes).

To be honest all heroes have potential but people always trying to abuse the system with easiest way.
It is the greed for winning the game.
And i also admit some heroes specially recent heroes skill set are so good that it seems op for us
But frankly speaking game itself is evolving.
And so do heroes also.
That is the reason it seems latest heroes are op.
Those heroes who has less counters or almost impossible to stop, you don't wanna face them.
That is the main reason of ban pick
And to answer your 2nd question, with proper strategy you can squize out 2 meta hero even if your team has 2nd pick. Because 2nd pick always 2 pick.
And we know 2 is better than one.....

i heard that there will be 6 banned slot soon...
yeah sometime i get annoying when my hero got banned, but it's more annoying when you play with the hero you cant stand in the game, for me sun user i always banned fanny hahahaah XD